Cold-formed High Strength Steel Research Articles

Web crippling behaviour of cold-formed high-strength steel unlipped channel beams under End-One-Flange load case

High-strength Cold-Formed Steel (CFS) members are widely adopted as structural members in building structures due to its higher ultimate capacity. The flexural members are often subjected to concentrated transverse loads which may leads to buckling instabilities including web crippling. However, there is no appropriate design rules and studies are available to estimate the web crippling strength of high-strength CFS members. Hence, this paper presents a detailed numerical investigation on high-strength CFS unlipped channel sections subjected to End-One-Flange (EOF) loading condition with nominal yield strengths of 700 MPa, 900 MPa and 1000 MPa. For numerical simulation study, non-linear Finite Element (FE) models were developed and validated with the experimental results followed by an extensive parametric study using ABAQUS. In total, 243 FE models were developed with different geometric and material parameters including section thickness, material strength, web slenderness ratio, inside bent radius to thickness ratio and bearing length to thickness ratio. The ultimate web crippling strength results were compared with the available design guidelines to check their suitability and accuracy in terms of strength prediction. Then, new design rules to predict the web crippling capacity of high-strength CFS unlipped channel section under EOF condition based on unified and Direct Strength Method (DSM) approaches were proposed.

Experimental investigation on material properties and residual stresses in cold-formed high strength steel irregular octagonal hollow sections

A systematic investigation into the variation of the material properties and residual stress distribution across cold-formed high strength steel (HSS) irregular octagonal hollow sections (IOctHS) is presented in this paper. The specimens were fabricated through welding the two cold-formed half-sections by gas metal arc welding (GMAW). Non-destructive inspection was executed to detect any cracks in the welding internally and externally. Tensile tests on coupon specimens taken from the critical locations within the cross sections were carried out to measure the material properties variation within the HSS IOctHS. The material strength variations within different cross sections exhibit an anticipated similar pattern that strength enhancement at corner regions was more obvious than flat coupons counterpart. The largest strength enhancement is 8.4% at corner region compared with the average yield strength of parent plates, whereas the yield strength and ultimate strength of flat coupon specimens generally fluctuate at a certain level. Based on the obtained material properties, the existing material models for predicting the stress-strain curves were assessed and new material models are proposed to generate the stress-strain relationship for materials within the HSS IOctHS. Moreover, the residual stress measurement for cold-formed HSS IOctHS was performed using the sectioning method. A total of 55 strip specimens were extracted from the cross sections of HSS IOctHS with more than 660 strain readings. Based on the residual stress measurement results, the magnitude and distribution pattern of the residual stresses are presented and discussed. A predictive model for residual stress distribution of the cold-formed HSS IOctHS was developed.

Local buckling resistances of cold-formed high-strength steel SHS and RHS with varying corner radius

This paper reports an experimental and numerical study of the cross-sectional behaviour of cold-formed high-strength steel (HSS) square and rectangular hollow section (SHS and RHS) members. Six stub column tests, six 3-point and three 4-point bending tests have been carried out, followed by the development of accurate finite element (FE) models and a parametric study. Based on the test and FE results, the design rules for slender cross-sections in European, American, Australian and Chinese standards and the GSRM method have been assessed via reliability analyses. A new equation for deriving the cross-sectional slenderness of SHS and RHS considering the influence of corner radius and new area reduction factors for designing slender cold-formed S900 (and above) SHS and RHS have also been proposed.

Elevated temperature and post-fire stress-strain modeling of advanced high-strength cold-formed steel alloys

Advanced high-strength cold-formed steels (AHSS) have been developed with yield strengths up to 1250 MPa and ultimate strengths up to 1900 MPa. Accurate stress-strain modeling of these new steel alloys are required for eventual application of AHSS into the building construction industry, including the material behavior of AHSS subjected to elevated temperatures. A series of new constitutive material models are proposed based on the experimental AHSS stress-strain database at elevated temperatures (i.e., steady-state and transient-state test methods) and post-fire scenario (i.e., residual test method). The stress-strain curves from experiments on two families of AHSS, including dual phase steel with nominal yield strengths of 340 MPa and 700 MPa, and martensitic steel with nominal yield strengths of 1030 MPa and 1200 MPa, reported in a previous study are considered. Existing stress-strain models are investigated and fit to the AHSS database, in addition to the development of new material models. The required input parameters of the proposed stress-strain models are determined, and their corresponding predictive expressions are proposed. It is shown the stress-strain behaviors of AHSS could be accurately described by the proposed models. The data generated by this research addresses fire safety design and will be essential in supporting the adoption of these next-generation steels in future infrastructure.

Strength predictions of circular hollow section T-joints of steel grade 1100 MPa

This paper presents experimental and numerical investigations of cold-formed high strength steel (CFHSS) circular hollow section (CHS) T-joints. The CFHSS CHS members have nominal 0.2% proof stress of 1100 MPa. The geometric parameters of the T-joints were designed by varying the ratios of β, 2γ and τ. A total of twelve T-joint tests were conducted by applying axial compressive load through the braces without preloading in chords. Non-linear finite element (FE) model was then developed for the CFHSS CHS T-joints. After successful validation, parametric studies were performed by using the verified FE model. The chord plastification failure of the T-joints was mainly found in both test and numerical studies. The relationship between the joint strengths and the variation of geometric ratios were investigated. The test and numerical strengths of the T-joints were used to assess the strength predictions by design equations provided in CIDECT, EN-1993-1-8 and the literature. It was found that these predictions generally provided unconservative predictions. A new equation that considers the effects of geometric ratios on the strengths was proposed based on both the test and numerical results. By adopting the newly proposed equation, the predictions are improved and provide the least scattered results when compared with other predictions.

Effect of member orientation on static strengths of cold-formed high strength steel tubular X-joints

Rotation of brace and/or chord members has a significant effect on the static strengths of tubular joints. Brace-rotated and bird-beak tubular joints are obtained by rotating the brace and/or chord members along their respective longitudinal axes by angles corresponding to their cross-sectional diagonal planes. This paper presents an experimental investigation of 29 cold-formed high strength steel (CFHSS) brace-rotated (BR), square bird-beak (SBB) and diamond bird-beak (DBB) tubular X-joints. The tubular members were made of S960 steel, having a nominal yield strength of 960 MPa. An axial compression load was applied through the braces of brace-rotated and bird-beak tubular X-joints. The welds connecting braces and chords of brace-rotated and bird-beak tubular X-joints were laid using the automatic gas metal arc welding process. The measured values of brace-to-chord width ratio (β) ranged from 0.33 to 0.83, effective brace-to-chord width ratio (β’) ranged from 0.25 to 0.87, brace-to-chord thickness ratio (τ) ranged from 0.67 to 1.28 and chord width-to-chord thickness ratio (2γ) ranged from 25.3 to 39.1. Three failure modes were observed in this study, namely chord face failure by plastification, a combination of this with chord sidewall failure (i.e., combined failure), and chord crown failure by plastification. Nominal strength prediction methods, from prior studies on member-rotated tubular joints as well as from the prEN 1993-1-8 (2021) and CIDECT (2008, 2009), were evaluated against the static strengths obtained from the tests. Reliability analysis was also performed to check the reliability levels of the existing design rules. For the range of tests undertaken, it was found that the current design rules of RHS-to-RHS, CHS-to-RHS and CHS-to-CHS tubular X-joints given in the CIDECT (2008, 2009), when used in conjunction with a material factor of 0.80, were found suitable and reliable for the design of CFHSS BR, SBB and DBB X-joints, respectively. In addition, the effect of change of member-rotation on the joint strengths and load–deformation curves was also assessed by comparing the structural performance of member-rotated tubular X-joints with the those of corresponding identical traditional RHS-to-RHS X-joints (Pandey and Young [1]).

Post-fire behaviour of cold-formed high strength steel tubular T- and X-joints

An experimental programme to investigate the post-fire residual strengths of cold-formed high strength steel (CFHSS) tubular T- and X-joints subjected to ISO-834 standard fire [1] exposure has been presented in this study. A total of 32 joints, including 16 T-joints and 16 X-joints, was tested. The tubular members of T- and X-joints were made of S900 and S960 steel grades. The test specimens were divided into 3 sets, and subsequently, these sets were then heated up to 300 °C, 550 °C and 750 °C by generally following the ISO-834 standard fire [1] curve, respectively. Once the fire exposed tubular T- and X-joints cool down to ambient temperature, tests were then conducted. The brace members of tubular T- and X-joints were made of square, rectangular and circular hollow sections (SHS, RHS and CHS), while SHS and RHS were used as the chord members. In this study, two configurations of tubular T- and X-joints were tested, first, where SHS and RHS were used as the brace and chord members (i.e. RHS-RHS), and second, where braces were made of CHS and chords were made of SHS and RHS (i.e. CHS-RHS). The T-joints were tested by applying axial compression load through the brace members and the chord ends were supported on rollers. The X-joints were tested by applying axial compression load through the brace members without chord preload. The welding was performed using automatic gas metal arc welding process. For T- and X-joints, the nominal values of brace-to-chord width ratio (β) ranged from 0.42 to 1, brace-to-chord thickness ratio (τ) ranged from 0.75 to 1, chord width-to-thickness ratio (2γ) ranged from 25 to 35 and chord side wall slenderness ratio (h0/t0) ranged from 15 to 35. Currently, there are no specific design rules to predict the post-fire residual strengths of tubular joints. Thus, the post-fire residual joint strengths obtained in this investigation were compared with the nominal strengths calculated from the CIDECT [2] and Eurocode 3 (EC3) [3] using the measured post-fire residual mechanical properties. It is shown that the current design rules of T- and X-joints given in the CIDECT [2] and EC3 [3] could not accurately predict the post-fire residual strengths of cold-formed S900 and S960 steel grades tubular T- and X-joints.

Ultimate resistances of member-rotated cold-formed high strength steel tubular T-joints under compression loads

Brace-rotated and bird-beak joints are novel tubular joint configurations, which are obtained by the rotation of rectangular and/or square hollow section (RHS and/or SHS) members along their respective longitudinal axes by angles corresponding to their cross-sectional diagonal planes. The load vs deformation behaviour and static joint resistances of these novel tubular T-joints are presented in this study. In total, 30 cold-formed high strength steel (CFHSS) member-rotated tubular T-joints were tested. In this paper, member-rotated joints include brace-rotated (BR), square bird-beak (SBB) and diamond bird-beak (DBB) configurations, where RHS and SHS were used as braces and SHS were used as chords. The steel grade of RHS and SHS members was S960. The joint resistances of test specimens were determined by applying axial compression loads through the braces and supporting the chord ends on rollers. The automatic gas metal arc welding process was used to connect braces and chords. The brace width-to-chord width ratio (β) ranged from 0.33 to 0.83, effective brace width-to-chord width ratio (β′) ranged from 0.25 to 0.88, chord width-to-chord thickness ratio (2γ) ranged from 25.3 to 38.9 and brace thickness-to-chord thickness ratio (τ) ranged from 0.67 to 1.28. Three failure modes were observed in this investigation, which include chord face failure (F), combined failure (F + W), and chord crown failure (C). The joint resistances were compared with the nominal resistances obtained from the literature and EC3 (2005, [1]). It has been demonstrated that the design rules available in the literature are generally not suitable for the design of CFHSS brace-rotated and bird-beak T-joints made of S960 steel. Further, the effect of change of member-rotation angle (ω) on the joint resistances of CFHSS T-joints made of S960 steel was observed by comparing the joint resistances of BR, SBB and DBB T-joints of this investigation with the joint resistances of traditional RHS T-joints (Pandey and Young (2019), [2]) made of identical tubular member dimensions and steel grade.

Stress concentration factors of cold-formed high strength steel tubular T-joints

A test programme looking into the stress concentration factors (SCF) of cold-formed high strength steel (CFHSS) traditional (without brace and/or chord rotation) and member-rotated tubular T-joints is described in this paper. The brace members of traditional T-joints were made of square, rectangular and circular hollow sections (SHS, RHS and CHS), while the brace members of member-rotated T-joints were made of SHS and RHS. On the other hand, the chord members of traditional and member-rotated tubular T-joints were made of SHS. In this study, member-rotated T-joints, include brace-rotated (BR), square bird-beak (SBB) and diamond bird-beak (DBB) tubular T-joints. The tubular members were cold-formed from thermomechanical control processed (TMCP) S900 and S960 steel grades plates. A total of 12 tests was conducted in this experimental investigation. An axial compression load was applied through the brace member, while the chord ends were supported on rollers. The brace and chord members of tubular T-joints were connected using the robotic gas metal arc welding process. The values of normal brace width-to-chord width ratio (β) ranged from 0.33 to 0.73, effective brace width-to-chord width ratio (β’) ranged from 0.40 to 0.87, brace thickness-to-chord thickness ratio (τ) ranged from 0.66 to 1.28 and chord width-to-chord thickness ratio (2γ) ranged from 20.6 to 39.0. The quadratic extrapolation method was used to estimate hot spot strains at the weld toes using the strains measured in the extrapolation regions. The experimental SCF values of CFHSS traditional and member-rotated tubular T-joints were compared with the SCF values obtained from the parametric equations given in the CIDECT (Zhao et al., 2001, [1]) and studies (Tong et al., 2013, [2]; Li et al., 2018, [3]; Tong et al., 2015, [4]). It was found that the existing SCF parametric equations given in the literature (Zhao et al., 2001, [1]; Tong et al., 2013, [2]; Li et al., 2018, [3]; Tong et al., 2015, [4]) could not accurately predict the SCF of cold-formed traditional and member-rotated tubular T-joints made of S900 and S960 steel grades.

Chord plastification in high strength steel circular hollow section X-joints: Testing, modelling and strength predictions

This paper presents experimental and numerical investigations on cold-formed high strength steel (CFHSS) circular hollow section (CHS) X-joints. The CFHSS CHS members had nominal 0.2% proof stress up to 1100 MPa. The geometric parameters of the X-joints were designed by varying the ratios of β, 2γ and τ. Seventeen X-joint tests were conducted by applying axial compressive load through the braces without preloading in chords. Non-linear finite element model (FEM) was then developed for the CFHSS CHS X-joints. After successful validation of ultimate strengths, failure modes and load-deformation curves, parametric studies were performed by using the verified FEM. The chord plastification failure of the X-joints was mainly found in both test and numerical studies. The relationship between the joint strengths and the variation of geometric ratios were investigated. The strengths of the X-joints obtained in this study together with the test strengths collected from the literature were used to assess the strength predictions by CIDECT and EN-1993–1-8, as well as those from the literature. It was found that the current predictions generally provided unconservative predictions. A new equation that considers the effects of geometric ratios on the strengths was proposed based on both the test and numerical results. By adopting the newly proposed equation, the predictions are improved and provide the least scattered results when compared with the other predictions.

Ductile fracture simulation of cold-formed high strength steel using GTN damage model

The use of high strength steel (HSS) cold-formed hollow section is of significant interest in the construction sector due to competitive costs. The accurate simulation of ductile fracture failure at the micro and macro scale is critical for improving the validity of predicting structural behaviour of HSS cold-formed hollow sections and welded joints. In this study, the ductile failure of the cold-formed S700 material is studied using the Gurson-Tvergaard-Needleman (GTN) damage model. Representative volume element (RVE) models with the void volume fraction (VVF) between 0.1% and 30% are employed to investigate the pressure dependency of the deviatoric limit stress. Different load conditions corresponding to different stress triaxiality levels are applied to unit cells with random spherical pores. The inelastic response of the unit cells are analysed, and the parameters q1, q2, and q3 in the GTN yield surface are calibrated. An equation is proposed to determine parameter q1 for different VVFs. The parameters critical VVF fc and final VVF ff are calibrated by the coupon test from the literature. Finally, the calibrated GTN model is validated against notched coupon tests. The finite element (FE) results show good agreement with the experimental results, indicating that the identified GTN model could efficiently predict the behaviour of the cold-formed HSS.

Experimental investigation on stress concentration factors of cold-formed high strength steel tubular X-joints

This paper describes a test programme to investigate the stress concentration factors (SCF) of cold-formed high strength steel (CFHSS) traditional (without brace and/or chord rotation) and member-rotated tubular X-joints. The braces of traditional tubular X-joints were made of square, rectangular and circular hollow sections (SHS, RHS and CHS), whereas chords were made of SHS and RHS. The braces of member-rotated tubular X-joints were made of SHS and RHS, whereas chords were made of only SHS. The member-rotated tubular joints include brace-rotated (BR), square bird-beak (SBB) and diamond bird-beak (DBB) tubular joints. The tubular members were cold-formed from thermomechanically control processed (TMCP) S900 and S960 steel grades plates. A total of 15 tests was conducted to investigate the SCF of CFHSS traditional and member-rotated tubular X-joints subjected to brace axial compression loads. The brace and chord members were joined together using the automatic gas metal arc welding process. The measured values of brace width-to-chord width ratio (β) ranged from 0.33 to 0.74, effective brace width-to-chord width ratio (β’) ranged from 0.25 to 0.85, chord width-to-chord thickness ratio (2γ) ranged from 20.5 to 38.7 and brace thickness-to-chord thickness ratio (τ) ranged from 0.66 to 1.27. The experimentally obtained SCF values were compared with the SCF values calculated from the CIDECT [1] and literature [2,3]. It has been demonstrated that the existing SCF parametric equations are generally quite conservative and over-predicted the SCF values for cold-formed traditional and member-rotated tubular X-joints of S900 and S960 steel grades. Moreover, in order to observe the effect of member-rotation(s), the maximum SCF values of the brace and chord members of CFHSS member-rotated tubular X-joints were also compared with the maximum SCF values of CFHSS traditional tubular X-joints.

Post-fire mechanical response of high strength steels

A test programme looking into the residual mechanical properties of fire exposed high strength steels (HSS) has been presented in this study. Accurate predictions of the residual mechanical properties of fire exposed structures are necessary to develop a proper strategy for their restoration. A test programme was conducted to investigate the post-fire residual material properties of thermomechanical control processed cold-formed HSS tubular members. The nominal yield strengths of tubular members were 900 and 960 MPa. The tubular members investigated in this study were made up of square, rectangular and circular hollow sections. The tubular members were heated inside a gas furnace, where the fire exposure generally followed the heating part of the ISO-834 standard fire [1] curve. The tubular members were exposed to 4 fire exposure temperatures, i.e. 300°C, 550°C, 750°C and 900°C, followed by their natural air-cooling inside the furnace. Subsequently, at ambient temperature, a total of 80 post-fire coupons was extracted from these ISO-834 standard fire exposed tubular members. The post-fire coupons were extracted from the flat, corner and curved regions of the fire exposed tubular members. The post-fire material properties and their respective residual factors were determined using the static stress–strain curves, which were obtained from their corresponding test stress–strain curves. New prediction equations are proposed for the post-fire residual material properties using the post-fire residual factors obtained in this test programme as well as available data in the literature for steel grades more than or equal to S690. It should be noted that two sets of prediction equations are proposed in this study, first, for the flat region, and second, for the corner and curved regions. The proposed prediction equations for the post-fire residual material properties of the flat region are valid for both hot-rolled and cold-formed HSS with nominal yield strengths ranged from 690 to 1070 MPa. Whereas, the proposed prediction equations for the post-fire residual material properties of the corner and curved regions are valid for cold-formed HSS with yield strengths ranged from 900 to 1200 MPa.

Static resistances of cold-formed high strength steel tubular non-90° X-Joints

This paper presents a test programme to investigate the static joint resistances and load vs deformation behaviour of cold-formed high strength steel (CFHSS) tubular non-90° X-joints. The investigation presented in this study predominantly focused on the chord failure of non-90° X-joints under brace axial compression loads. The experimental programme comprised of 26 tubular non-90° X-joints, which were fabricated with three included angles (θ1), i.e. 30°, 50° and 70°. Circular and square hollow sections were used as braces, while chords were made up of rectangular and square hollow sections. The nominal yield strengths of the tubular members were 900 and 960 MPa. The brace and chord members were connected using automatic gas metal arc welding process. The measured value of brace-to-chord width ratio (β) ranged from 0.53 to 1, brace-to-chord thickness ratio (τ) ranged from 0.52 to 1.01, chord width-to-thickness ratio (2γ) ranged from 20.5 to 38.7 and chord side wall slenderness ratio (h0/t0) ranged from 15.4 to 38.9. A total of three failure modes was observed, including chord face failure, chord side wall failure and combination of these two failure modes. The static joint resistances obtained from the tests were compared with the nominal joint resistances of X-joints given in the CIDECT and Eurocode 3. It is shown that the existing design rules in these specifications are not suitable for the design of S900 and S960 steel grades tubular non-90° X-joints. Further, the impact of the included angle (θ1) on the joint resistances of X-joints was observed by comparing the joint failure resistances of tubular non-90° X-joints obtained in this investigation with the joint failure resistances of tubular 90° X-joints of identical nominal tubular member dimensions and steel grades. In addition, tensile material properties from the weld heat affected regions of S960 steel grade tubular members of different thicknesses were also investigated.

Numerical investigation and design of fully chord supported tubular T-joints

A numerical investigation focusing on the design of fully chord supported cold-formed high strength steel (CFHSS) tubular T-joints is presented in this paper. The nominal yield strengths of the tubular members were 900 and 960 MPa. Square, rectangular and circular hollow sections (SHS, RHS and CHS) were used as the brace members, while SHS and RHS were used as the chord members. The tests conducted by Pandey and Young (2019) [1] were used to calibrate finite element (FE) models developed in this numerical investigation. The developed FE models were capable of replicating the test results, including joint resistances, load-deformation histories and failure modes. In order to determine the compression resistance, axial compression loads were applied through the brace members by fully supporting the chord members along their lengths. These calibrated FE models were then subsequently used for the parametric study, which comprised of a total of 189 FE analyses on T-joints with SHS and RHS braces and 96 FE analyses on T-joints with CHS braces. The nominal resistances calculated from the EC3 (2005) [2] and CIDECT (2009) [3] were used to compare the compression resistances obtained from both the tests [1] and the parametric study. In order to check the applicability of the design rules of tubular T-joints, the nominal resistances from these specifications (EC3 (2005) [2] and CIDECT (2009) [3]) were obtained with and without incorporating the recommended reduction (or material) factors. Through the comparison of 307 joints, including 22 tests (Pandey and Young (2019) [1]) and 285 FE analyses, it has been demonstrated that the existing design rules of T-joints given in the EC3 (2005) [2], CIDECT (2009) [3] cannot be directly used for fully chord supported tubular T-joints of S900 and S960 steel grades with validity ranges of governing parameters exceeding the limits specified in these specifications (EC3 (2005) [2], CIDECT (2009) [3]). Hence, new semi-empirical design rules are proposed in this paper for the observed failure modes, including chord face failure, chord side wall failure and their combined failure modes. Reliability analyses were also performed to check the reliability levels of both the existing and proposed design rules.

Design of cold-formed high strength steel tubular T-joints under compression loads

This study presents a comprehensive finite element (FE) analysis of cold-formed high strength steel (CFHSS) tubular T-joints. The brace members of tubular T-joints were made up of rectangular (including square) and circular hollow sections (RHS and CHS), whereas the chord members were made up of RHS. The nominal yield strengths (i.e. 0.2% proof stresses) of the tubular members were 900 and 960 MPa for S900 and S960 steels, respectively. Finite element (FE) models were developed and verified against the tests conducted by Pandey and Young (2019), showing the capability of reciprocating the experimental joint strengths, failure modes and load-deformation histories. The material properties and test results used for the validations of the FE models are reported in Pandey and Young (2019). The tubular T-joints were tested under axial compression through the brace members, while the ends of the chord members were supported on rollers. Upon validations of the FE models, a parametric study comprised of 285 FE analyses was carried out. The validity ranges of governing parameters in this study exceeded the current validity ranges given in the EC3 (EN 1993-1-8, 2005) and CIDECT (Packer et al., 2009). A total of 309 joint strengths obtained from the tests (Pandey and Young, 2019) and parametric study were compared with the nominal strengths obtained from the EC3 (EN 1993-1-8, 2005) and CIDECT (Packer et al., 2009). In this study, three failure modes were observed, namely chord face failure, chord side wall failure and combined failure. The applicability of current chord stress function given in the CIDECT (Packer et al., 2009) was also evaluated for cold-formed tubular T-joints of S900 and S960 steel grades. It is shown that the existing design rules given in the EC3 (EN 1993-1-8, 2005) and CIDECT (Packer et al., 2009) are not directly suitable for T-joints of S900 and S960 steels with validity ranges of governing parameters exceeding the limits specified in these specifications (EN 1993-1-8, 2005, Packer et al., 2009) and their modifications are needed. Therefore, using two approaches, i.e. semi-empirical and by applying correction factors on the latest equations given in the EC3 (EN 1993-1-8, 2005, EN 1993-1-12, 2007), design rules are proposed in this study for cold-formed tubular T-joints of S900 and S960 steel grades. In addition, reliability analyses were also performed to check the reliability levels of the existing and proposed design rules.

Mechanical properties of cold-formed steel cladding profiles at elevated temperatures

Cold-formed steel (CFS) claddings are increasingly used in bushfire prone areas in Australia due to their non-combustibility and are recommended in the Australian standard AS3959 for the design of buildings in bushfire prone areas. Thus, good knowledge and understanding of their behaviour at elevated temperatures is important for the safe design of buildings. However, the elevated temperature mechanical properties of thin CFS cladding profiles have not been investigated. Hence, this research investigated the elevated temperature mechanical properties of high strength CFS cladding profiles through steady-state tensile coupon tests. More than 100 coupons, taken from the crests of three different cladding profiles with thicknesses of 0.42 mm and 0.48 mm, were tested to failure at predetermined elevated temperatures. The elevated temperature mechanical property results were compared with corresponding results from previous studies conducted on CFS sheets and sections, and the predictions obtained from the Australian CFS design standard. These comparisons showed that currently available equations can be used to determine the elevated temperature stress–strain curves and mechanical properties of CFS cladding profiles except for ultimate and fracture strains. Since no suitable equations are available for ultimate and fracture strains of CFS at elevated temperatures, new equations are proposed for CFS with varying thicknesses.

Structural performance of concrete-filled cold-formed high-strength steel octagonal tubular stub columns

This paper presents a comprehensive experimental and numerical study on the structural performance of concrete-filled cold-formed high-strength steel octagonal tubular stub columns. Stub column specimens formed using high-strength steel and infilled concrete with grades C50 and C90, and with three different plate width-to-thickness ratios were tested. The ultimate loads, load-displacement responses and failure modes of the structures were observed and discussed. In addition to the experimental investigations, a finite element model validated using the stub column test results was developed. A series of parametric studies were subsequently conducted to obtain supplementary data for concrete-filled cold-formed high-strength steel octagonal tubular stub columns with a wide range of plate width-to-thickness ratios and different concrete compressive strengths. The applicability of existing design approaches in European and American standards and in literature provided for concrete-filled steel tubular structures with rectangular, circular or octagonal cross-sections were evaluated using the results obtained from the experimental and numerical study. The accuracy of design predictions for concrete-filled cold-formed high-strength steel octagonal tubular stub columns using different design approaches was discussed. A design approach that more accurately incorporates the strength contributions from steel tubes and concrete infill was also proposed for structural design.

Development of explicit DSM for cold-formed channels under combined bending and shear

High strength cold-formed steel channel beams widely used in roof and floor systems carrying steel profiled roof sheeting or floor decking are commonly subjected to a combined action of bending and shear in practice. The design guidelines of such purlins or beams have recently been standardised utilising shear-bending interaction curves in current standards/specifications such as the Australian/New Zealand Standard (AS/NZS 4600:2018) and the North American Specification (NAS S100–2016). In order to investigate different combinations of bending and shear, an experimental program comprising thirty tests of pure bending, combined bending and shear and predominantly shear has been performed at the University of Sydney using a dual actuator test rig. All the tests are conducted on commercial plain C-lipped and Supa-Cee channels. The interaction between bending and shear is governed by varying ratios of bending moments at two ends of the test span. All testing results are plotted against the current interaction curve to validate the performance of the current design method on channel members under different combinations of bending and shear. Further, a completely new concept for the interaction between bending and shear on a theoretical basis of statics is introduced and a development of an explicit Direct Strength Method (DSM) design for the prediction of ultimate strengths of members under combined bending and shear is also provided in this study.

Effect of bending process on the fatigue behaviour of high strength steel

Integrating forming in the fatigue analyses of cold-formed High Strength Steel parts significantly increases predictive accuracy. In this study, the effect of cold-bending is modelled through a coupled forming-fatigue simulation of a bent specimen, made from a 12 mm thick S500MC grade. Different strain hardening models were studied, where a Voce law led to an increase in prestrain of about 10% and a decrease in fatigue life of 20% compared to the reference hardening behaviour. Three bending radii were investigated and a 25% decrease in radius led to a 30% increase in plastic strain. Fatigue tests were performed and a local strain approach resulted in a 76% correspondence with experimental data.